A display device of the type mentioned in the opening paragraph is known from the international patent application US2006/0227196. This patent application discloses an electronic ink display which comprises two substrates. One of the substrates is transparent and is provided with a single electrode which is referred to as the counter electrode. This counter electrode is in other literature also referred to as common electrode or backplane electrode. The other substrate is provided with picture electrodes which comprise row and column electrodes. A display element or pixel is associated with an intersection of a row electrode and a column electrode. A pixel electrode of the display element is coupled to the column electrode via a thin film transistor (further referred to as TFT), the gate of which is coupled to the row electrode. This arrangement of display elements, TFT transistors and row and column electrodes together forms an active matrix. A row driver sequentially selects rows of display elements and the column driver supplies data signals to the selected row of display elements via the column electrodes and the TFT transistors. The data signals correspond to graphic data to be displayed.
An electronic ink is provided between the pixel electrode and the common electrode. The electronic ink comprises multiple microcapsules of about 10 to 50 microns. Each microcapsule comprises positively charged white particles and negative charge black particles suspended in a fluid. When a positive voltage is applied to the pixel electrode with respect to the common electrode, the positively charged white particles move to the side of the micro capsule directed to the transparent substrate on which the common electrode is present and a viewer will see a white display element. Simultaneously, the black particles move to the pixel electrode at the opposite side of the microcapsule where they are hidden to the viewer. By applying a negative voltage to the pixel electrode with respect to the common electrode, the black particles move to the common electrode at the side of the micro capsule directed to the transparent substrate and the display element appears dark to a viewer. When the voltage is removed, the display device remains in the acquired state and thus exhibits a bi-stable character. The electronic ink display with its black and white particles is particularly useful as an electronic book.
Grey scales are created in the display device by controlling the amount of particles that move to the common electrode at the top of the microcapsules. For example, the energy of the positive or negative electric field in the pixel caused by the voltage difference between the pixel and common electrodes, defined as the product of field strength and time of application, controls the amount of particles moving to the top of the microcapsules.
A multi-level drive of the pixel electrodes enables to more closely reach a desired light output of a pixel because it is possible to more accurately control the movement of the particles at a lower value of the drive voltage. However, such a multi-level drive of the pixel electrodes requires complex display drivers.